The present invention relates generally to filters for use inside blood vessels. More particularly, the present invention relates to thrombus filters which can be securely affixed at a selected location in the vascular system and removed when no longer required.
There are a number of situations in the practice of medicine when it becomes desirable for a physician to place a filter in the vascular system of a patient. One of the most common applications for vascular filters is the treatment of Deep Venous Thrombosis (DVT). Deep Venous Thrombosis patients experience clotting of blood in the large veins of the lower portions of the body. These patients are constantly at risk of a clot breaking free and traveling via the inferior vena cava to the heart and lungs. This process is known as pulmonary embolization. Pulmonary embolization can frequently be fatal, for example when a large blood clot interferes with the life-sustaining pumping action of the heart. If a blood clot passes through the heart it will be pumped into the lungs and may cause a blockage in the pulmonary arteries. A blockage of this type in the lungs will interfere with the oxygenation of the blood causing shock or death.
Pulmonary embolization may be successfully prevented by the appropriate placement of a thrombus filter in the vascular system of a patient""s body. Placement of the filter may be accomplished by performing a laparotomy with the patient under general anesthesia. However, intravenous insertion is often the preferred method of placing a thrombus filter in a patient""s vascular system.
Intravenous insertion of a thrombus filter is less invasive and it requires only a local anesthetic. In this procedure, the thrombus filter is collapsed within a delivery catheter. The delivery catheter is introduced into the patients vascular system at a point which is convenient to the physician. The delivery catheter is then fed further into the vascular system until it reaches a desirable location for filter placement. The thrombus filter is then released into the blood vessel from the delivery catheter.
In the treatment of Deep Venous Thrombosis, a thrombus filter is placed in the inferior vena cava of a patient. The inferior vena cava is a large vessel which returns blood to the heart from the lower part of the body. The inferior vena cava may be accessed through the patient""s femoral vein.
Thrombus filters may be placed in other locations when treating other conditions. For example, if blood clots are expected to approach the heart and lungs from the upper portion of the body, a thrombus filter may be positioned in the superior vena cava. The superior vena cava is a large vessel which returns blood to the heart from the upper part of the body. The superior vena cava may by accessed through the jugular vein, located in the patient""s neck.
Once placed inside a blood vessel, a thrombus filter acts to catch and hold blood clots. The flow of blood around the captured clots allows the body""s lysing process to dissolve the clots.
The walls of the blood vessels are lined with a thin inner membrane or intima. When the anchor portions of a thrombus filter puncture this inner membrane the body responds in a process referred to as neointimal hyperplasia. As a result, the punctured area of inner membrane is overgrown with a number of new cells. The anchor portions of the thrombus filter are encapsulated with new cell growth (neointimal hyperplasia).
Due to neointimal hyperplasia, thrombus filters placed in the blood vessel of a patient become affixed in the blood vessel walls within two weeks after being implanted. Because the portions of the filter contacting the blood vessel wall become fixed in this way, many prior art filters cannot be removed percutaneously after being in place for more than two weeks.
The present invention pertains to a thrombus filter and a method of removing a thrombus filter using minimally invasive methods while limiting complications due to neointimal encapsulation of the anchor portions of the thrombus filter. A thrombus filter for use with the inventive removal method includes a body member and a plurality of elongated struts. Each strut has a joined end and a free end. The joined end of each strut is fixably attached to the body member on the thrombus filter. The struts radiate outwardly from the body member of the thrombus filter such that the thrombus filter is generally conical in shape. A thrombus filter includes an insulating layer substantially covering the thrombus filter including the body portion, struts, and anchor members. The insulation layer includes an opening. In one embodiment of the current invention, the opening in the insulation layer is proximate a sacrificial link. The opening in the insulation layer allows direct contact between a portion of the thrombus filter and the patient""s blood. This direct contact creates a conductor path between the thrombus filter and the body of the patient.
When removal of a thrombus filter is desired, a first electrical conductor forms an electrical connection with the body portion of the thrombus filter. A second electrical connector forms an electrical connection with the patient""s body. A power supply is used to selectively apply a voltage differential between the body portion of the thrombus filter and the body of a patient. This voltage differential induces a current through the thrombus filter.
When a voltage differential is applied between the body portion of the thrombus filter and the body of a patient, current flows between the thrombus filter and the patient""s blood at a location proximate the opening in the insulating layer. The flow of electrical current between the thrombus filter and the patient""s blood causes electrolytic corrosion of the thrombus filter in a selected area. The current flow is continued until a selected area of the thrombus filter has been weakened or cut through by electrolytic corrosion. In one embodiment of the thrombus filter, electrolytic corrosion weakens or cuts through a sacrificial link holding an anchor to the thrombus filter.
When the thrombus filter is deployed inside a blood vessel, the free ends of the struts engage the blood vessel wall. The body member of the thrombus filter is held in a position proximate the center of the blood vessel by the plurality of struts which engage the blood vessel walls with opposing force vectors. The conical formation of struts acts to trap or capture blood clots. The generally conical shape of the formation of struts, serves to urge captured blood clots toward the center of the blood flow. The flow of blood around the captured blood clots allows the body""s natural lysing process to dissolve the clots.
To assure firm attachment of the thrombus filter to the blood vessel, anchor members may be attached to the free ends of the struts. In one embodiment of the thrombus filter, anchors are attached by a sleeve. In a second embodiment of the thrombus filter, anchor members are attached by a sacrificial link. In a third embodiment of the thrombus filter, the anchors are formed from the strut itself.
When anchor members are attached by a sacrificial link, the material of the sacrificial link is selected to be one which is susceptible to electrolytic disintegration in blood. The thrombus filter includes an insulating layer substantially covering the thrombus filter including the body portion, the struts and the anchor members. An opening in the insulation layer is selectively created over a portion of the thrombus filter which retains anchors to the free ends of the struts. This opening in the insulating layer allows direct contact between the material of the thrombus filter and the patient""s blood. This direct contact creates a conductive path between a portion of the thrombus filter and the patient""s blood.
When removal of the thrombus filter is desired, a removal catheter with a lumen and a distal end is disposed in the lumen of the blood vessel. The removal catheter enters the patient""s vascular system at a point which is readily accessible to the physician. Once in the vascular system, the catheter is urged forward until the distal end of the removal catheter is proximate the thrombus filter. A first electrical conductor is disposed inside the lumen of the removal catheter. The first electrical conductor includes a proximal end and a distal end. The distal end of the first electrical connector penetrates the insulating layer on the thrombus filter to form an electrical connection with the body portion of the thrombus filter. The proximal end of the first electrical conductor is electrically connected to a power supply. A proximal end of the second electrical conductor is also electrically connected to the power supply. The distal end of the second electrical conductor is electrically connected to the body of the patient.
The power supply is used to selectively apply a voltage differential between the first electrical conductor and the second electrical conductor. This voltage differential induces a current flow between the thrombus filter and the blood of the patient. Because the thrombus filter is covered with an insulating layer, this current must flow through the area proximate the opening in the insulating layer. The flow of current causes electrolytic corrosion of the thrombus filter in the selected area. The flow of electrical current between the thrombus filter and the patient""s blood is continued until a selected portion of the thrombus filter has been weakened or cut completely free by electrolytic corrosion. The weakened link will separate when a load is applied, for example, by an is elongated pulling tool.
When the anchor portions of the thrombus filter are separated in this fashion, the thrombus filter may be pulled into the lumen of the retrieval catheter leaving the anchor portions attached to the blood vessel wall. Pulling the thrombus filter into the lumen of the retrieval catheter causes the struts to collapse. When the struts collapse, the thrombus filter is transformed from a generally conical shape to a generally cylindrical shape. Pulling of the thrombus filter into the lumen of the retrieval catheter may be accomplished with a retrieval wire including a hook. The anchor members remain fixed in the walls of the blood vessel by encapsulating cell growth due to neointimal hyperplasia. When the thrombus filter is pulled into the lumen of the retrieval catheter, the removal of the thrombus filter from the patient becomes simply a matter of withdrawing the removal catheter from the lumen of the blood vessel.